The present invention generally relates to gas generating systems, such as on-board inert gas generating systems (OBIGGS) and, more particularly, to air separation modules (ASM) for gas generating systems.
The Federal Aviation Agency (FAA) has issued safety mandates requiring that all new and existing aircraft inert the fuel tank to prevent explosions. One accepted method of fuel tank inerting is the OBIGGS. Aircraft have used the OBIGGS to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen enriched air (NEA).
A key component of the OBIGGS is the ASM. The ASM may be configured to receive air, such as bleed air, and to provide a NEA flow to inert the fuel tank and an oxygen enriched air (OEA) flow. The OEA flow can be recaptured or vented overboard.
The ASM can include a housing having an inlet to receive air (e.g. bleed air), a NEA outlet and an OEA outlet. The ASM may include a bundle of hollow fiber membranes positioned within the housing. At least one end of the fiber bundle may be cast or potted in what is commonly referred to as a tubesheet. More commonly, both ends of the fiber bundle may be so encapsulated with the tubesheet. The tubesheets may serve to hold the fibers in a fluid-tight relationship such that the NEA flow may be isolated from the OEA flow. An inner tube, such as the inner tube described in US Patent Application 2010/0024649 A1, may be included within the fiber bundle to provide axial support.
The ASM utilizes membrane technology to separate oxygen from air. During operation, air may enter the housing through the inlet and pass through the bundle of hollow fiber membranes. Oxygen may be separated from the air flow due to diffusion through the fiber walls because the fiber walls may be more permeable to oxygen than to nitrogen. As the air flow travels through the ASM, the NEA flow may be generated by the loss of oxygen via permeation through the fiber wall. The NEA flow may exit the housing through the NEA outlet and the OEA flow may exit through the OEA outlet. Typically, an NEA oxygen concentration of 12% or less is sufficient to inert the fuel tank.
In fuel tank inerting systems, the amount of NEA flow may be the key performance metric. Higher NEA flow per size and weight of ASM translate into lower the OBIGGS size and weight. Further, it is desirable that the ASM have a long maintenance interval to reduce cost and delays associated with the servicing of the ASM.
As can be seen, there is a need to increase the NEA flow of the ASM. Further, there is a need to reduce the weight, size and maintenance interval of the ASM.